To determine whether increased intracellular levels of magnesium ([Mg]i) are associated with enhanced functional capacity, we performed symptom-limited exercise treadmill testing on 42 stable coronary artery disease (CAD) patients (37 men, 5 women, mean age 68 ± 9 years). [Mg]i was found to be an independent and significant predictor of exercise duration (R = 0.31, p = 0.02) in a multivariate stepwise regression model. Patients with > normal [Mg]i of 1.23 µg/mg protein (n = 13) had a significantly greater mean functional capacity, measured in higher achieved metabolic equivalents (10.6 ± 2.5 vs. 8.9 ± 2.3, p < 0.05) and exercise duration (9.4 ± 2.3 vs. 7.9 ± 2.2 min, p < 0.05) compared to patients with [Mg]i ≤ the normal (n = 29). Thus, functional capacity is greater in stable CAD patients with higher [Mg]i, suggesting that magnesium may play a role in CAD pathophysiology, possibly via ventricular unloading.